Current sanitary materials (water absorbent articles) such as a disposable diaper, a sanitary napkin, and an incontinence pad typically include, as a constituent ingredient (water absorbing agent), a water absorbent resin and a hydrophilic fiber such as pulp to absorb body fluids.
Known examples of the water absorbent resin as a water absorbing agent include a crosslinked partially neutralized polyacrylic acid, a hydrolyzed starch-acrylonitrile graft polymer, a neutralized starch-acrylic acid graft polymer, a saponified vinyl acetate-acrylic ester copolymer, a crosslinked carboxymethyl cellulose, a hydrolyzed acrylonitrile copolymer or hydrolyzed acrylamide copolymer, a crosslinked acrylonitrile copolymer or crosslinked acrylamide copolymer, a crosslinked cationic monomer, a crosslinked isobutylene-maleic copolymer, a crosslinked polymer of 2-acrylamide-2-methylpropanesulfonate and acrylic acid and the like.
Recent years have seen a tendency toward an increase in (i) the amount (g) of use of a water absorbent resin in a single sanitary material such as a disposable diaper and a sanitary napkin and in (ii) the weight ratio (weight %) of a water absorbent resin relative to the entire absorbent body including the water absorbent resin, a hydrophilic fiber and the like. Specifically, (i) the amount of a hydrophilic fiber (pulp), which has a low bulk specific gravity, is reduced while (ii) the use amount of a water absorbent resin, which has an excellent water absorbency and a high bulk specific gravity, is increased for an increase in the proportion (weight %) of a water absorbent resin in an absorbent body. This intends to reduce the thickness of sanitary materials without decreasing the amount of water that the sanitary materials are capable of absorbing. This has in turn required a water absorbent resin, instead of a hydrophilic fiber such as pulp, to serve a function related to liquid transportation and allocation.
A gel of a water absorbent resin swollen in an absorbent body serves a transportation function with use of a capillary phenomenon through gaps between gel particles. It is typically presumed that a water absorbent resin with higher gel strength has higher transportability, while a water absorbent resin with lower gel strength has lower transportability, that is, lower diffusibility of liquid in an absorbent body, as a result of a so-called gel blocking phenomenon.
Typically, to have high gel strength in a swollen state, a polymer for the water absorbent resin should have a high degree of crosslinking. Such a high degree of crosslinking, however, inevitably leads to a decrease in the swelling capacity and retention ability. Patent Literature 1 discloses a method for improving the swelling pressure of a gel of a surface-crosslinked water absorbent resin produced through an acid-type polymerization at a low neutralization rate and subsequent neutralization. This method is, however, problematic in that, for example, it jeopardizes the safety in handling a polymer with high acidity and complicates the production process. The above method is therefore difficult to apply to industrial production.
A water absorbent resin may, as is well-known, be subjected to a surface treatment to have high gel strength. This technique causes a water absorbent resin to be treated with use of (i) any of various surface-crosslinking agents that can react with a carboxyl group in polymer molecules at the surface of the water absorbent resin or (ii) a particular polymer that can react as such, and thus intends to produce a water absorbent resin with high gel strength and high liquid-absorbing ability under pressure. The technique thereby prevents a gel blocking phenomenon.
Various attempts have been made at such surface treatments to modify the surface of a water absorbent resin for prevention of a gel blocking phenomenon. Known examples of such attempts include a method of using a water absorbent resin crosslinked with use of a particular metal ion (Patent Literatures 2 and 3), a method of modifying a water absorbent resin with use of a polyamine and polyimine in an organic solvent (Patent Literature 4), a method of surface-treating a water absorbent resin with use of a surface treatment agent containing a polyol and a cation in the state of an aqueous solution (Patent Literature 5), a method of surface-treating a water absorbent resin with use of (i) an organic crosslinked compound other than a polyol and (ii) a surface treatment agent containing a cation in the state of an aqueous solution (Patent Literature 5), and the like. Any of these publicly known methods may be used to prevent gel blocking.
The above publicly known methods, however, fail to ensure sufficient transportability of liquid in an absorbent body. Further, surface-crosslinking a water absorbent resin for increased gel strength increases the crosslinking density at the surface and its vicinity of the particles, so it fails to essentially improve the gel strength as the inside of the particles remains untreated.
There have already been particularly well-known attempts of adding, for example, an inorganic compound to a water absorbent resin to improve usability, preservability, or water-absorbing performance of a powder of the water absorbent resin. Known examples of such attempts include a method of dry-blending a polyhydric metal salt such as aluminum sulfate with a water absorbent resin and subsequently contacting the blended product with a binding agent (for example, water) to produce a water absorbent resin in which gel blocking does not easily occur (Patent Literature 6), a method of mixing in a Vortex mixer a water absorbent resin with a permeability retaining agent (for example, silica, alumina, titania, clay, an emulsified polymer, or a precipitated polymer) and subsequently applying mechanical stress to the mixture in, for example, an Osterizer blender (Patent Literature 7), a method of coating with a three-dimensional or electrostatic spacer a surface-crosslinked water absorbent resin having a particular gel strength (Patent Literature 8), a super water absorbent resin composition containing a fine powder of an aggregate of a hydrous oxide including a super water absorbent resin and two kinds of metals M1 and M2 each having a -M1-O-M2- bond as at least part thereof (Patent Literature 9), and the like.
While the above publicly known methods can prevent gel blocking, even the use of the above methods may fail to achieve sufficient transportability of liquid in a diaper. Even if the above methods achieve sufficient liquid transportability, they require an excessively large amount of a gel-blocking preventing agent such as organic or inorganic fine particles. The above methods thus pose such problems of dust as dusting, filter clogging, and the like in a production line for a water absorbing agent or a diaper. The above methods therefore leave room for improvement in terms of safety and cost.
There has also been proposed, as a method for improving the gel strength of a water absorbent resin while maintaining its swelling capacity and retention ability, a method of using a chain transfer agent in combination for polymerization and using a crosslinking agent in a large amount (Patent Literature 10). However, this method, which not only adds a chain transfer agent but also requires a crosslinking agent in an amount larger than normal, is disadvantageous in terms of cost.
Another known example of an index of the degree of gel blocking is saline flow conductivity (SFC) in Patent Literature 11. Examples of a method of adding inorganic fine particles to improve the SFC and other liquid permeabilities include the methods disclosed in Patent Literatures 12 to 17, etc. These evaluation methods, however, also fail to produce a water absorbent resin that achieves sufficient performance when used in a diaper.
Other known techniques for improving a water absorbent resin are (i) techniques in which the particle size of a main component is 600 to 150 μm (Patent Literatures 17 to 19) and techniques in which the particle size of a main component is 297 to 149 μm (Patent Literature 20).
The above methods have improved and dealt with such physical property values of a water absorbent resin as not only water absorption capacity (without load), water absorption capacity under load, water absorbing speed, and liquid permeability, but also particle size, moisture content, degree of coloration, and the like. There has been proposed a water absorbent resin of which the above physical property values are controlled. For example, as disclosed in Non Patent Literatures 1 to 7 listed below, physical property values such as water absorption capacity, water absorption capacity under load, and water absorbing speed are defined by, for example, the European Diaposables And Nonwovens Association (EDANA) and JIS (the JIS defines the water absorption capacity and water absorbing speed of a water absorbent polymer). Specifically, the water absorbing speed (typically the Vortex method or FFST method) is defined on the basis of the absorption time (seconds), that is, the time period necessary for a dry powder of a water absorbent resin to gelatinize a predetermined amount of liquid. Thus, the water absorbing speed is used to evaluate the behavior of a dry powder of a water absorbent resin.
As described above, although there have been proposed a lot of improvements for water absorbent resins, there is still room for improvement in the absorbing ability of a water absorbent resin in an absorbent article (for example, a diaper). Specifically, a diaper requires such properties as “absolute absorption amount (g)” (indicative of the absorbing ability of a single diaper). For an increasing in the “absolute absorption amount (g)”, a water absorbent resin simply needs to have increased water absorption capacity (centrifuge retention capacity or CRC (g/g)). However, increasing the water absorption capacity (CRC) tends to, for the second and third instances of urine discharge in a diaper, decrease the “liquid absorbing time (in particular, a liquid absorbing time for the second time and thereafter) and also decrease the “diffusion distance (%)” (index of liquid diffusibility). In addition, increasing the “absolute absorption amount (g)” does not necessarily improve the “re-wet (g)” of a diaper.